Apparently an ideal ammeter has zero resistance. But for a series circuit, why does it matter? The purpose of an ammeter is to give the reading for the amount of current going round a 'loop' of circuit. Having some resistance does not prevent the ammeter from giving a true reading, right? So if this ammeter is connected with a resistor, and the ammeter gives a reading of 10A, that is the true value of current going through the resistor. So if you're trying to do an experiment to get the IV (current-voltage) curve of a component, the plotted curve would look the exactly the same regardless of whether you've used a zero resistance ammeter or not, right?